In order to enhance off-road driving performance of a four-wheel drive vehicle, it is common to install a ‘limited slip’ differential, instead of a conventional ‘open’ differential, between two respective wheel-axle shafts. The limited slip differential mechanically limits relative rotation of the wheel-axle shafts according to a defined bias, therefore allowing up to a pre-determined difference in wheel speed. This proves useful when the vehicle is driven across loose or uneven road conditions where one of the wheels is unable to maintain grip, for example, by coming into contact with ice, causing friction to decrease and the wheel to spin. When this occurs, the limited slip differential diverts useful torque away from the wheel-axle connected to the spinning wheel, to the opposite wheel-axle, provided there is some friction between the connected wheel and the ground. This allows the opposite wheel to grip and propel the vehicle over the problematic road condition. However, as limited slip differentials allow some relative rotation of the wheel-axle shafts, there are occasions where the diverted torque is not sufficient for the opposite wheel to grip and negotiate the problematic road condition, which can cause the driver to lose control of the vehicle.
In order to further enhance the off-road driving performance of a four-wheel drive vehicle, it is common to install a ‘locking’ differential between two respective wheel-axle shafts. The locking differential allows the wheel-axle shafts to be operatively ‘locked’ and effectively act as a single shaft, thereby preventing relative rotation of the wheel-axle shafts and eliminating difference in wheel speed. This is particularly useful when road conditions are very loose and the wheels are unlikely to be able to maintain grip. By preventing relative rotation of the wheel axles, traction is significantly increased, allowing the vehicle to traverse the loose road conditions.
One drawback to locking differentials is the decrease in vehicle handling performance. When a locking differential is operated, it often proves difficult to steer the vehicle, as negotiating a turn typically requires one wheel to rotate faster than the other, and therefore the differential is typically deactivated prior to turning. However, if the locking differential is deactivated whilst turning across loose road conditions, meaning that the differential reverts to behaving as an ‘open’ differential, this can result in a significant decrease in traction and potentially cause the driver to lose control of the vehicle.
It would be useful to provide a combination of a locking and limited slip differential, to provide optimum traction during all driving conditions. One example of such a differential is disclosed in U.S. Pat. No. 5,865,701. The disclosed locking limited slip differential uses a ‘clutch pack’ comprising a stack of friction disks (as typically found in limited slip differentials) to limit relative rotation of a pair of wheel-axles or alternatively, to prevent relative rotation altogether. However, whilst this differential may function adequately, it is foreseeable that the differential also has a number of drawbacks. For example, as the mechanism for ‘locking’ the wheel-axles is a conventional clutch pack and the torque values typically involved in locking wheel-axles of four-wheel drive vehicles are much higher than those involved in limiting slip between wheel-axles (torque is in the order of 1000 Nm for locking compared to around 100-200 Nm for limiting slip), this is likely to result in significantly greater friction forces and heat energy being generated in the clutch pack, therefore causing greater wear and likely decreasing the useful life of the differential.
Accordingly, it would be advantageous to provide a locking limited slip differential which is less prone to damage during locking operations and therefore more durable than prior art approaches. Furthermore, it would be useful to provide a solution that avoids or ameliorates any of the disadvantages present in the prior art, or which provides another alternative to the prior art approaches.
Any discussion of documents, acts, materials, devices, articles or the like included in the present specification is not to be taken as an admission that any or all of these matters form part of the common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application.